Chapter 4 Geologic Time: Concepts

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Chapter 4 - Geologic Time Concepts Principles

• Introduction John Wesley Powell expeditions
  through the Grand Canyon 1869 first record-ed
  passage through the canyon. 1871 exped-ition
  included a photographer, a surveyor, and three
  topographers. John Wesley Powell bio
• Grand Canyon is fascinating because of the
  exposure of a long period of geologic time.
• Relative dating Geologic events placed in
  order.
• Absolute dating Specific dates for rock units
  or events.

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Grand Canyon Layers
Permian Kaibab Limestone (250 m.y.)
Middle Cambrian Tapeats Sandstone (545 m.y.)
Grand Canyon Supergroup (1.25 b.y. to
825 m.y.)
Brahma Schist/ Vishnu Schist/ Zoroaster
Granite(2.0 b.y. to 1.7 b.y.)
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Tapeats Sand-stone is the oldest, flat-lying
sedimentary rock unit in the Grand Canyon. Was
deposited during Cambrian flooding of
conti-nental margins, after two periods of
geologic uplift.
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• Concepts of time and the changes thereof
• St. Augustine of Hippo (354 to 430 A.D.) helped
  establish the concept of linear time, B.C. and
  A.D. time scale.
• Bishop James Ussher (1600s) estimated Earths
  creation took place on 10/22/4004 B.C., used
  recorded history and Genesis geneology.
• John Joly (1899) estimated 90 million years for
  oceans to reach current salinity.

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Concepts of relative dating (17th 18th
centuries) Superposition (Nicolas
Steno) Original horizontality (Steno) Lateral
continuity (Steno) Cross-cutting relationships
(Hutton) Inclusions (James Hutton) Fossil
succession (Wm. Smith) Sedimentary sequences
(Smith) Uniformitarianism (Hutton/Chas.
Lyell) Establishes a Time Line without knowing
the absolute ages (radiometric dates).
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Numerous geologic concepts are on display in the
Grand Canyon.
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Cross-cutting relationships Inclusions - James
Hutton
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Cross-cutting relationships
Lava flow
Fault
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Volcanic and Sedimentary Inclusions
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Xenoliths are inclusions of pre-existing rocks
into magmas.
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• Neptunism Catastrophism Biblically-based
  concepts of Earth creation
• Neptunism Abraham Werner (1787) suggested
  that all rocks precipitated from global flood
  (seawater), even igneous rocks (see p. 61, Table
  4.1).
• Catastrophism Georges Cuvier (late 18th
  century) suggested Earths landforms were due
  to a series of catastrophies.
• Assumptions were abandoned for lack of field
  evidence.

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Uniformitarianism James Hutton (late 18th
century) and Charles Lyell (19th century) the
present is the key to the past. Hutton saw the
Earths history as cyclical, mountains are
eroded, redeposited, uplifted to form new
mountains. He understood gaps in the rock
record (Fig. 4.4) Actualism modern viewpoint
uniformit-arianism occasional catastrophes.
Lord Kelvin vs. Charles Lyell text discussion
on p. 63, efforts, modified uniformitarianism to
allow for cooling of a molten Earth.
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USGS Numeric Time Scale also see Figure 4.1, p.
58.
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• Radioactive isotopes, with known decay rates
  parent/daughter products are used for absolute
  dating.

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• Atom the smallest particle of an element with
  all of the characteristics of that element.
  Atoms consist of a nucleus composed of protons
  and neutrons and a shell of orbiting electrons.
  Atoms are neutrally charged.
• Isotopes - Variations of atoms with one or more
  extra neutrons, resulting in slightly different
  characteristics.
• Atomic number - of protons. Atomic mass - of
  protons and neutrons.
• Types of radioactive decay on page 64.

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Half-Life the period of time needed for ½ of
a radioactive parent product to decay to its
daughter product. Generally, radiometric age
dating is most useful for igneous rocks. The
particular parent/daughter products used for
age-dating depend on whats there and the
estimated age range. Decay rates are constant,
but heating may remobilize daughter products,
resetting the radiometric clocks.